Wavelength selective radiation responsive systems and devices



Nov. 24, 1959 E. E. LoEBNER 2,914,679

WAVELENGTI-l SELECTIVE RADIATION RESPONSIVE SYSTEMS AND DEVICES FiledApril 5, 1956 gdnE Iaabzwr f 2,914,679 Patented Nov. 24, V1959WAVELENGTH SELECTIVE RADIATION RESPON- SIVE SYSTEMS AND DEVICES Egon E.Loebner, Belle Meade, NJ., assignor to Radio Corporation of America, acorporation of Delaware Application April 5, 1956, Serial No. 576,260

13 Claims. (Cl. Z50-213) This invention relates to systems utilizingradiations of Various wavelengths, and in particular to systems anddevices adapted to respond to radiations in selected spectral regionsonly.

Materials which have the property of changing their electrical impedancein response to the incidence of radiations are well known in the art andare collectively referred to as photoconductors. This property may beobserved by placing a photoconductive material between two electrodesand applying a potential difference between such electrodes. It will befound that the photoconductive material has a very high electricalimpedance when the photoconductive material is shielded from al-lradiations. However, when radiations, visible or invisible, to which thelmaterial is sensitive are allowedr to impinge upon the photoconductivematerial the impedance of such material, will be found to decreasesubstantially. It will also be found that such photoconductive responseof the photoconductive material `is dependent upon the intensity fiersand radiation reproducing and storage devices of various kinds. In allof such devices, the variation of the impedance of the photoconductivematerial in accordance with the intensity of radiations incident thereonhas been utilized. Therefore, the primary objective of the art has beento produce and utilize photoconductorshaving a response covering a widespectral region, or, in otherwords, photoconductors which will respondto as many different wavelengths as possible. response has the efIect ofincreasing the sensitivity of the photoconductors since low levels ofradiations of a great vnumber of diiferent wavelengths will produce thesame result as a high level of radiations of a single wavelength.

Thus, the wavelength selectivity of photoconductors,

although known, has not been exploited to any great eX- tent.Photoconductive materials having a broad spectral region of responsehave been used torespond to two or more different wavelengths ofradiation. More recently the selectivity of photoconductors has beenused to enable the amplification of a certainwavelength of radiationindependently of other wavelengths or the conversion of certainwavelengths of radiation intorother wave-` lengths. However, thewavelength selectivity of photoconductive response has not been'utilizedin its bestadvantage. j K

It is an object Vof this invention `to provide devices capable ofproducing radiations in response to the in- Such wide band 2 cidence ofany one or all of two or more selected spectral regions only ofradiation;

It is a further object of this invention to provide devices capable ofproducing or storing radiations in a selected spectral region inresponse to the incidence of any one or all of two or more selectedspectral regions only of radiation.

It is still another object of this invention to provide new and usefulsystems and devices wherein information is carried by combinations ofdistinct spectral regions of radiation.

Brielly, one embodiment of this invention comprises a rst radiationresponsive element, the electrical impedance of which is decreased bythe incidence thereon of radiations in a given spectral region only, anda voltage responsive element electricallyconnected in series with suchirst radiation responsive element. A second radiation responsiveelement, the electrical impedance of which is decreased by the incidencethereon of radiation only in another spectral region different from saidgiven spectralV region,` is directly electrically connected in parallelwith the first radiation responsive element and in series with thevoltage responsive element. In operation the incidence of radiation ineither spectral region will reduce the impedance of one of the radiationresponsive elements thus causing the voltage responsive element to beactuated. 'According to one feature of this invention, a `singlephotoconductive element, adapted to have a -maximum response in two ormore different spectral regions only, may be used in place of theparallel connected photoconductive elements.

A modification 'according to this invention may comprisein addition toone or more of the embodiments, above described, a source of radiationsmatched to each ofthe spectral regions involved and which is adapted tobe modulated to provide 'radiations in dierent spectral regions inaccordance with certain desired information, the voltage responsiveelement providing an output of a desired nature.

t This invention will be more completely understood l when the followingdetailed description is read in conluminescent radiation curves for anumber of well-known electroluminescent materials;

Figure 6 is a cross-sectional view of a device constructed in accordancewith the subject invention; and

Figure 7 is a cross-sectional view of a storage device constructed inaccordance with this invention.

Referring to Figure l there is shown a pair of photoconductive orradiation responsive elements or cells 12 and 14 electrically connectedin parallel with each other, and a load 16 electrically connected inseries with the parallel connected elements or cells 12 and 14. When avoltage source 18 is connected across the series parallel circuit, itwill be seen that the voltage will be divided between the load 16 andthe parallel connection of the radiation-responsive elements 12 and 14.As is characteristic of parallel circuits, the voltage appearing there`across will be determined primarily by the element 12 or 14 having `thelowest impedance and its relative magnitude as compared to that of theload 16. t may be assumed that when the radiation responsive elements 12and 14 are shielded from all radiations they will have approximately thesame very high impedance, as is characteristic of such materials.. Thus,if the impedance of the load is much smaller than either of thempedances of the elements 12 and 14 the voltage appearing across theload 16 will be some small portion of the applied voltage. and the load16 may be chosen or adapted to be insensitive to such small voltage.

In operation, radiations C1 to which one of the radiation responsiveelements 12 or'14 is sensitive are caused to impinge upon such elements.Similarly, radiations C2 to which the other responsive element issensitive are caused to impinge on said other element. Such radiationswill cause a substantial decrease in the impedance of the selectedelementand thus a decrease in voltage appearing across the parallelconnected elements 12 and 14 and an increase inA the voltage appearingacross the load 16. Thus, the load 16 may be adapted to be excited bysuch increase in voltage, or at such increased voltage, to give adesired output. It will be seen that the incidence of radiation uponeither of the radiation responsive elements 12 or 14 will produce theabove described redistribution of voltages in the circuit. It will alsobe seen that the incidence of radiations C1 and C2 upon both of theradiation responsive elements 12 and 14 will produce a correspondingredistribution of voltages in the circuit.

According to this invention each of the' radiation responsive elements12 and 14 is composed of a material which is sensitive to radiations ina given spectral region only. Thus, where the spectral region in whichthe rst element 12 is sensitive does not appreciably overlap thespectral region in which the second element 14 is sensitive, an outputmay be induced only by the incidence thereon of radiations in either oneor both of the two spectral regions. Referring to Figure 2, there isshown a graph of the generalized response curves of anumber of knownradiation responsive materials. The radiation or photoconductiveresponse of the materials is plotted against the wavelength of theincident radiations. It will be seen from the graph of Figure 2 that anumber of combinations of materials could be selected for use as theradiation responsive elements 12 and 14. Thus, the embodiment of thisinvention shown in Figure 1 may be adapted'to respond to radiations ineither one or both oftwo different spectral regions but not tovradiations in any other spectral region. Such a device may be used todetect the presence of radiations in either or both spectral regions andto give an output in accordance therewith.`

The radiation responsive cells or elements 12 and 14 may each comprise alarge crystal of a radiation responsive material to` which twoelectrodes are attached, as is known in the art. Each of such elements12 and 14 may also comprise radiation responsive material in the form ofcrystals of powder particle dimensions bound together by a suitablematrix into a layer or stratum and sandwiched between two electrodes, orthey may comprise a layer of radiation-'responsive material sinteredonto one electrodeand another electrode in contact therewith, such asare described in applications of Frederick H. Nicoll, Serial No.527,476, led August l0, 1955 and now abandoned, and Soren M. Thompson,Serial No. 473,001, filed December 3, 1954 now U.S. VPatent number2,765,381.

Certain structural and electrical advantages may be obtained through theuse of a single radiation responsive element having a response inVeither of two different spectral regions, in -place of the two separateelements hereinabove described. For example, two layers, as

described above, each of a dilferent radiation responsivel material, maybe sandwiched, side by side, between the is shown.

4 same two conductors, in which case the element as a whole wouldexhibit the response of both materials.

Figure 3 shows a single cylindrical crystal 20 having a response ineither of two diierent spectral regions, which may also be used toreplace the parallel connected radiation responsive elements. Such acrystal may be produced by modifying the spectral response of a crystal'22 of a first radiation responsive material by forming on at least onesurface thereof a layer 24 of a second radiation responsive material inthe same crystal structure as that of the irst material. A preferredmethod for modifying the spectral response of a cadmium sulde crystal,for example, comprises heating a crystal thereof supported over aquantity of selenium at a temperature of about 700 C. The seleniumvolatilizes and reacts with the cadmium sulfide forming a layer 26 ofcadmium sulfoselenide in a surface layer. A similar process may becarried out in the production of a sintered layer on an electrode, inwhich case a sintered layer would be produced having a response similarto that of the single crystal 20 described above.

Referring to Figure 4, a graph of the photoconductive response of thecrystal 20 plotted against wavelength It will be seen that the responsecurve has two Vpeaks of response, one in each of two different spectralregions. However, it will also be seen that the crystal exhibits alesser degree of response in the spectral region between such peaks.Such intermediate response is probably due to the response of theinterface region 26 between the separate crystal and the surface layerthereon. The response curve of the crystal 22 has been completed indotted lines in Figure 4 as has been the response curve for the surfacelayer 24. The curve consisting entirely of dotted lines is believed torepresent the response of the interface region 26 mentioned above. It isdesirable to reduce the response curve of Vsuch interface region 26 to anegligible value in order to enhance the selectivity of the crystal withrespect to spectral response. Such reduction can probably beaccomplished by adjusting the process to minimize the thickness of suchinterface region 26. However, the effects of such intermediate responsecan also be minimized through the proper design and operation of thedevice, as described hereinafter.

The operation of an embodiment of this invention is complicated by thefact that the magnitude of the response of a radiation responsivematerial is a function of both the wavelength Vof the incidentradiations and the intensity of such radiations. The graphs shown inFigures Zand 3 are normalized graphs representing the response of thevarious materials when the intensity of the various wavelengths ofradiations are held constant. Thus, it will be seen that high intensityradiations of a wavelength to which a given material is relativelyinsensitive may produce the same response as low intensity radiations ofa wavelength to which the material is moresensitive. p Furthermore,since the graphs shown in Figures 2 and 4 were evolved by Vsubjectingthe various radiation responsive materials, sequentially, to diierentradiations each in a very narrow band, they do not necessarily representthe response `which the radiation responsive material would show to eachwavelength of radiation if all of the wavelengths of the radiations werepresent on the material at the same time. Thus, it will be seen thatthis invention does not ,completely eliminate the necessity forradiation shielding 'in all embodiments. To state the matter morebroadly, it is not the primary purpose of this invention to eliminatethe necessity of radiation shields in radiation responsive devices.Embodiments of this invention must be very carefully designed andoperated in order to take advantage of the selectivity of thephotoconductive materials as represented by the graphs shown in Figures2 and 4.

The design considerations in embodiments of this invention are thusdivided into three main areas. First, of

course, isthe selection of pairs 0i rg-IUPSV of Photocon ductivematerials which have a very marked wavelength selectivity in spectral`regions that do not appreciably overlap eachother. AThe secondconsideration is the selection and adaptation ofa load -that willenhance the selectivity lof the radiation responsive material. Since allradiation responsive materials have a certain level of conduction evenin the dark (i.e. without the incidence of radiations) the load must notrespond to such dark level conduction. Similarly, the load must notrespond to the slight changes in the impedance of the radiationresponsive materials that might occur when radiations bordering onorslightly overlapping with the region of spectral response of theVmaterial impinge upon such material. Thus, it will be seen that the loadmay ybe adapted to enhance the selectivity of the radiation responsivematerial by designing it to respond only when a certain maximum ofimpedance decrease (or radiationA response) occurs in the radiationresponsive material.

The third area of design consideration is concerned with the radiationswhich are allowed to impinge upon the radiation responsive materials.The ideal situation would be to have a source or sources of radiationsin a plurality of spectral regions which are exactly matched to thespectral regions of response, respectively, of the radiation responsivematerials. Thus, if the radiation responsive materials were properlychosen, the overlap of the spectral regions of response, and thus, ofthe spectral regions of the radiations would be such that each radiationresponsive element would respond only kto the radiation having aspectral region matching its own and not to the other radiations. Thus,shielding would only be necessary to protect the radiation responsiveelements from external or ambient radiations.

According to this invention, electroluminescent elements are veryadvantageously used either as the load for the radiation responsiveelements or as a source of radiationsV for activating the radiationresponsive elements,` or both. Electroluminescent elements comprise alayer or stratum of certain well-known phosphors between a pair ofelectrodes. The application of a voltage to such electrodes will producean electric iield between the electrodes and through the phosphor layer.Such electric eld will induce radiations from the electroluminescentphosphor. It is known that an electroluminescent cell may be adapted tohave a certain threshold voltage below which no appreciable radiationswill be produced and above which radiations will be produced and whichwill vary in intensity with the magnitude of such'voltage. It will beseen that by adjusting the threshold voltage of the electroluminescentelement, a certain amount of selectivity may be obtained.

Furthermore, it is known that different electroluminescent materialswill electroluminesce in diterent spec tral regions. `In fact, it hasbeen found that the spectral regions of certain electroluminescentmaterials correspond favorably to the spectral regions of response ofcertain of the radiation responsive materials. Theresource elements 36and 38, The spectral response of each of thefradiation responsiveelements 32 and 34 are matched respectively to the spectral regions ofelectroluminescence of each of the electroluminescent source elements 36and 38. It will be' seen that at any given level of radiation intensityfrom the source elements 36 and 38 the eiliciency of operation will bedetermined by the degree of matching of the spectral regions of responseand electroluminescence.

A structure according to this embodiment may com prise a first sheet ofglass 40 upon one surface of which a transparent conductive coating 42has been formed as by the deposition of the vapors of stannic acid,water and methanol thereon. The electroluminescent load 30 may be placedin contact with the transparent conductive coating and may comprise alayer of particles of zinc sulfide with activator proportions of copperand coactivator proportions of iodine suspended in a transparentelectrical insulating material such as ethyl cellulose. A secondtransparent conductive coating 44 may be applied to theelectroluminescent load.30 and may comprise a thin layer of silver pastefor example. The radiation responsive elements 32 and 34 may be appliedto the second transparent conductive coating 44 in the form of adjacentlayers or coatings each covering a portion of the area of theelectroluminescent load 30. The radiation responsive elements maycomprise photoconductive crystals of powder particle dimensions boundtogether by a suitable matrix, or layers of photoconductive materialsintered in situ. The first radiation responsive element 32 may :becomposed of zinc sulde and the second radiation responsive element 34may be composed of cadmium sulde, for example. VA second sheet of glass46 having a transparent conductive coating 48 formed on one majorsurface thereof, as described with respect to the rst sheet of glass, ispositioned with the coating 48 in electrical contact with both radiationresponsive elements 32 and 34. The opposite major surface of the secondsheet of glass 46 is provided with two spaced and electrically insulatedconductive coatings 50 each corresponding in area and position to one ofthe radiation responsive elements 32 and 34. The pair ofelectroluminescent source elements 36 and 38 may be applied, one to eachof the conductive coatings 50 and coextensive therewith, in the for-rnof layers or coatings of electroluminescent material as described withreference to the electroluminescent load 30. The source element 36,which is coupled to the first radiation re'- sponsive element 32, may becomposed of electroluminescent boron nitride, for example, and thesource element 38, which is coupled to the second radiation responsiveelement 34, may be composed of (ZnS:Cu,Cl) zinc sulde with activatorproportion of copper and coactivator pro- 1 portions of chlorine, forexample. A conductive coatfore, ideal sources of radiations are providedby electroluminescent materials for embodiments of this invention.Referring to Figure 5, a graph is shown which represents the spectralregions covered by the `radiations from certain electroluminescentmaterials. By comparing the graph of Figure 5 with the graph of Figure 2it will be seen that certain of the spectral regions `ofelectroluminescence substantially duplicate certain of the spectralregions vof response of the radiation responsive materials.

Referring to Figure 6 an embodiment of this invention is shown in whichelectroluminescent materials are used both as the load and as sources ofradiation in two different spectral regions. An electroluminescent loadelement 30 is arranged in electrical contact with a pair of radiationresponsive elements 32 and 34. The radiation responsive elements are linturn arranged in radiation receiving relationship toa pair ofelectroluminescent ing 52 may then be applied to, and extending over,both electroluminescent source elements 36 and 38. A difierent Voltagesource 54- and 56 may be connected between the conductive coating 52 andeach of the conductive coatings S0 on the opposite side of the sourceelements 36 and 3S and switching means 58 and 60 may be provided toenable the energization of either of the source elements 36 and 38separately. Another voltage source 62 may be connected between the rsttransparent conductive coating 42 and the -third transparent conductivecoating 48.

It will be seen that the spectral region of electroluminescencc of theboron nitride material of which the rst source element 36 is composedsubstantially matches the spectral region of response of the zincsulfide material of which the rst radiation responsive element 32 iscomposed and that the spectral region of electroluminescence of theZnS:Cu,Cl material of which the second source element 38 is composedsubstantially matches thespectral region of response of the cadmiumsulfide Y'materialrof the .second-radiation responsive Yelement 34.

It will'be.seen,.furthermore thatthevspectralV region ofelectroluminescence-of the loa'd 30 is. intermediate the spectral,regions Vof. response of the radiation responslve elements 32 and 34.

Inoperation, if either of the switches 58 or 60 are closed,-,the sourceelement 36 and. 38 corresponding thereto will be turned .on andradiation therefrom will impinge upon the radiation responsive elements`32 and 34. The impedance of. only one radiation responsive element willbe appreciably decreased by radiations from each of the vsource elements36 and 38, however, due to the spectral region matching above described.Such decrease in impedance of oneof the radiation responsive elements 32and .3.4 willresult in a change in the voltage appearing -across the,electroluminescent load 3@ and Will cause an Velectroluminescent outputtherefrom. Furthermore, the electroluminescence emitted by the load 30will not affect the impedance of the radiation responsive elements 32kand 34 due to the mismatch of spectral regions described above.

'Thus, it will be seen that by proper adjustment of the thickness of thevarious layers and the voltages applied thereacross, this embodiment ofthe invention may be adapted to produce radiations only in response tothe energization of either one or both of the source elements.Similarly, proper mismatching of spectral regions may be used to modifythe output of the structure, as desired.

Referring to Figure 7, astructure according to another embodiment ofthis invention is shown. The device shown in Figure 7 is a storagedevice which makes use of the parallel connection, or the double peakresponse of a radiation responsive material according to this invention.The device shown in Figure 7 may comprise a rst sheet of glass 64 havinga transparent conductive coating 66 applied to one surface thereof byany suitable method, for example, by the deposition of the vapors ofwater, methanol and stannic acid thereon. A given electroluminescentmaterial may then be applied to such transparentconductive coating inthe form of a layer 68. For example, the electroluminescent material maycomprise particles of zinc sulde with activator proportions of copperand coactvator proportions of chlorine and may be suspended in atransparent insulating material, such as ethyl cellulose, for example,and sprayed or painted or otherwise applied to such transparentconductive coating. A second sheet of glass 70 may be provided with atransparent conductive coating 72 upon which a sintered layer 74 ofradiation responsive material having a double peak response similar tothat shown in Figure 4 may be applied, for example. The two layers 68and '74 are then placed in contact with each other and a voltage sourceis connected between the two transparent conductive coatings 66 and 72.It will be seen that'no radiations will be induced in theelectroluminescent material so long as there are no radiations incidentupon the layer 74 of radiation responsive material. However, if thedevice is exposed to radiations C1 in a spectral region to which theradiation responsive material 74 is sensitive, electroluminescence willbe induced in the electroluminescent material. According to one featureof the structure shown in Figure 7, the electroluminescent material maybe chosen so that the spectral region of the radiations C2 producedthereby corresponds to one of the spectral regions in which theradiations responsive material is sensitive. ness of the layers 68 and74 is properly adjusted to enable a gain of more than unity due to thefeedback of the radiation C2 from the electroluminescent material to theradiation responsiveimaterial, storage can be obtained. That is, onceradiations have been induced from the electroluminescent material inresponse to the incidence of radiations Ain a spectral region to whichthe radiation responsive.mater'ial is sensitive, such radiation willcon- Thus, if the thick-v tinuez even .after the.actuating radiationsare no Ylonger incident upon. the Vradiation responsive -materiaL due toYthefeedbackof radiations from theelectroluminescent material tothe:radiation. responsive material. Thus,` -it will.be..seen -thata'dev'iceisf provided -which is adapted to be actuated.by.radiations inonespectral regionsand Yto store inradiations Vof another spectralregion but will not react to ra'diationssin` any other spectral regioneven one-Which may be intermediate that of the actuating radiations yandthat .of thestorage-radiations.

Itwill be seen that there has been-provided hereinr'newelectroluminescent devicesand systems which are capable of making useof4 the wavelength selectivev properties of radiation responsivematerialin combination with certain light sources to.enablevthetranslating ofinformation'in the form of radiations of .various wavelengths into anoutput of ade'sired type. Suchl systems and devices will have wide usein .certain .storage applications wherein it is desired to store giveninformation or images. Furthermore, such vdevices and systems will beuseful in logic and calculator systems.l .According to thisinvention,the amount of information'which may be carried in agivenradiation beam may be increased inaccordance with a number of .spectralregions which are used.

Whatis claimed is:

1. vA wavelength-selective radiation responsive VIdevice comprising afirst sheet of transparent insulating'material, two spaced, transparentconductive coatings on' one major surface of said sheet, two elements ofelectroluminescent material, each of Vsaid `elements ofelectroluminescent material being in electrical contact with only one ofsaid coatings, an electrical .conductor extending over both of saidelements of electroluminescence material and in electrical contacttherewith, a transparent conductive coating on the other major surfaceofV said Vfirst sheet, two elements of radiation responsive material onsaid conductive coating on said vother surface of said sheet, each ofsaid elements of radiation responsive material corresponding in size andposition to said elements of electroluminescent material, anelectricalconductor extending over said elements of radiation responsive materialand in electrical contact therewith, a-layer of electroluminescencematerial on saidconductor extending over said elements of radiationresponsive material, and a second sheet of glass having a transparentconductive coating on one major surface thereofV positioned'withsaid-conductive coating in electrical contact with -said layer ofelectroluminescent material, each of said elements Vofelectroluminescent material being adapted to emit a different spectralregion only fof radiations andeach of said elements of radiationresponsive elements being-adapted to respond to a ,different spectral.region only of radiations, said spectral regions of saidelectroluminescence being matched to saidspectralregions of `response ofcorrespondingly positioned elements of electroluminescent material andradiation responsive material, respectively.

2., An electrical apparatus comprising a source of radiations in each ofva plurality of different spectral regions, radiation responsive meansin radiation receiving relationship with said source of radiations, saidradiation responsive means exhibiting a decrease in lelectricalirnpedance inresponse to radiations in said plurality of different-spectral regions only, and electroluminescent means, having a certainthreshold voltage above which a desired-output will occur, saidradiation. responsive means beingelectrically connected to saidelectroluminescent means in such manner that said electroluminescentmeans will remit light over its entire emitting area when said radiationresponsive means is excited in any of said spectralregions, each of saidplurality of spectral regions Vand the response of said radiationresponsive means ,and -said threshold-voltage of saidvelectroluminescent means being such as to provide a desired output forthe vincidence of any. one'of'said plurality of spectral regions: ofradiations on .said .radiation responsivey means.

3. A wavelength selective radiation responsive device comprisingelectroluminescent means and radiation responsive means, said radiationresponsive means having a variable impedance characteristic in responseto radiant energy and having pronounced maximum response in at least twodifferent spectral regions las compared to a substantial spectral regionof substantially lower response intermediate said regions of maximumresponse, said radiation responsive means being electrically connectedto said electroluminescent means in such manner that saidelectroluminescent means will emit light over its entire emitting areawhen said radiation responsive means is excited in either of saiddierent spectral regions.

4. A device as in claim 3, wherein said radiation responsive meanscomprises a plurality of radiation responsive elements connected inparallel with each other and having said pronounced maximum responses indierent spectral regions.

5. A device as in claim 3, wherein said radiation responsive meansincludes a single body of material having a double peaked response.

6. A wavelength selective radiation responsive device comprising anelectroluminescent element, rst and second photoconductive elements,conductive means electrically connecting said photoconductive lelementsin parallel with each other and in series with said electroluminescentelement so that said electrolumi-nescent element will emit light wheneither of said photoconductive elements is excited, said rstphotoconductive element having a maximum photoconductive response in onerelatively narrow band of wavelengths of light only, and said secondphotoconductive element having a maximum photoconductive responsesubstantially only in a different relatively narrow band of wavelengths.

7. A Wavelength selective radiation responsive device comprising a pairof spaced apart sheet-like electrodes, a layer of electroluminescentmaterial intermediate said electrodes, and photoconductive layer meansintermediate said electroluminescent layer and one of said electrodes,said photoconductive layer means having pronounced maximumphotoconductive response in at least two different spectral regions ascompared to a substantial response in the spectral region ofsubstantially lower response intermediate said regions of maximumresponse, said photoconductive layer means being electrically connectedto said electroluminescent layer in such manner that saidelectroluminescent layer will emit light over its entire emitting areawhen said photoconductive layer means is excited in either of saidspectral regions.

8. A device as in claim 7, wherein said photoconductive means comprisestwo layers of different material arranged along side each other.

10 9. A device as in claim 7, wherein said photoconductive meanscomprises a single body of material having a double peaked response.

10. A device as in claim 9 wherein said photoconductive material isresponsive to light falling within two relatively narrow bands ofwavelengths only, and said electroluminescent material is adapted toemit light falling substantially within one of said wavelength bandsonly.

ll. A Wavelength selective radiation responsive device comprising a pairof spaced apart sheet-like electrodes, a layer of electroluminescentmaterial intermediate said electrodes, iirst and second photoconductivelayer means intermediate said electroluminescent layer and one of saidelectrodes and arranged along side each other, said rst photoconductivelayer having a maximum photoconductive response in a rst relativelynarrow band of wavelengths only and said second photoconductive layerhaving a maximum photoconductive response in a second and differentnarrow band of wavelengths only, a iirst electroluminescent celladjacent to said iirst photoconductive layer and capable of emittinglight which is matched only to the response of said firstphotoconductive layer, and a second electroluminescent cell adjacent tosaid second photoconductive layer and capable of emitting the lightwhich is matched only to the response of said second photoconductivelayer.

`12. A device is in claim 1l, Wherein said electroluminescent layer isformed of a material which emits light falling outside said first andsecond narrow bands.

13. A device as in claim 3, wherein said electroluminescent means isadapted to emit light falling within said intermediate region.

References Cited in the ile of this patent UNITED STATES PATENTS2,641,712 Kircher June 9, 1953 2,742,550 Jenness Apr. 17, 1956 2,768,310Kazan et al Oct. 23, 1956 2,779,811 Picciano et al I an. 29, 1957FOREIGN PATENTS 157,101 Australia June 16, 1954 OTHER REFERENCESOrthuber et al.: A Solid-State Image lntensier, Journal of the IOpticalSociety of America, vol. 44, No. 4, pp. 297-299, April 1954.

Quarterly Review No. 3, Fellowship on Computor components #347, Melloninstitute of Industrial Research, pp. W-9, X41-10, Figs. VI-4, Vil-5.Date 1951.

1. A WAVELENGTH-SELECTIVE RADIATION RESPONSIVE DEVICE COMPRISING A FIRST SHEET OF TRANSPARENT INSULATING MATERIAL, TWO SPACED, TRANSPARENT CONDUCTIVE COATINGS ON ONE MAJOR SURFACE OF SAID SHEET, TWO ELEMENTS OF ELECTROLUMINESCENT MATERIAL, EACH OF SAID ELEMENTS OF ELECTROLUMINESCENT MATERIAL BEING IN ELECTRICAL CONTACT WITH ONLY ONE OF SAID COATINGS, AN ELECTRICAL CONDUCTOR EXTENDING OVER BOTH OF SAID ELEMENTS OF ELECTROLUMINESCENCE MATERIAL AND IN ELECTRICAL CONTACT THEREWITH, A TRANSPARENT CONDUCTIVE COATING ON THE OTHER MAJOR SURFACE OF SAID FIRST SHEET, TWO ELEMENTS OF RADIATION RESPONSIVE MATERIAL ON SAID CONDUCTIVE COATING ON SAID OTHER SURFACE OF SAID SHEET, EACH OF SAID ELEMENTS OF RADIATION RESPONSIVE MATERIAL CORRESPONDING IN SIZE AND POSITION TO SAID ELEMENTS OF ELECTROLUMINESCENT MATERIAL, AN ELECTRICAL CONDUCTOR EXTENDING OVER SAID ELEMENTS OF RADIATION RESPONSIVE MATERIAL AND IN ELECTRIAL CONTACT THEREWITH, A LAYER OF ELECTROLUMINESCENCE MATERIAL ON SAID CONDUCTOR EXTENDING OVER SAID ELEMENTS OF RADIATION RESPONSIVE MATERIAL, AND A SECOND SHEET OF GLASS HAVING A TRANSPARENT CONDUCTIVE COATING ON ONE MAJOR SURFACE THEREOF POSITIONED WITH SAID CONDUCTIVE COATING IN ELECTRICAL CONTACT WITH SAID LAYER OF ELECTROLUMINESCENT MATERIAL, EACH OF SAID ELEMENTS OF ELECTROLUMINESCENT MATERIAL BEING ADAPTED TO EMIT A DIFFERENT SPECTRAL REGION ONLY OF RADIATIONS AND EACH OF SAID ELEMENTS OF RADIATION RESPONSIVE ELEMENTS BEING ADAPTED TO RESPOND TO A DIFFERENT SPECTRAL REGION ONLY OF RADIATIONS, SAID SPECTRAL REGIONS OF SAID ELECTROLUMINESCENCE BEING MATCHED TO SAID SPECTRAL REGIONS OF RESPONSE OF CORRESPONDINGLY POSITIONED ELEMENTS OF ELECTROLUMINESCENT MATERIAL AND RADIATION RESPONSIVE MATERIAL, RESPECTIVELY. 